listener: support multi-accept like nginx

While accepting a single connection at-a-time is likely best for
multi-worker and/or load-balanced deployments; accepting
multiple connections at once should be less bad on overloaded
single-worker systems.

We can't automatically pick the best value here since worker
counts are dynamic via SIGTTIN/SIGTTOU.  Process managers
(e.g. systemd) can also spawn multiple instances sharing a
single listener with no knowledge sharing between listeners.

diff --git a/Documentation/public-inbox-daemon.pod b/Documentation/public-inbox-daemon.pod
index 81a79a102511a054cb2b60c623f97836f5c6d89b..7121683325c78eebe4bd86da3e47984f9fbacc53 100644 (file)
--- a/Documentation/public-inbox-daemon.pod
+++ b/Documentation/public-inbox-daemon.pod
@@ -115,6 +115,23 @@ per-listener C<cert=> option.  The private key may be
 concatenated into the path used by the cert, in which case this
 option is not needed.
 
+=item --multi-accept INTEGER
+
+By default, each worker accepts one connection at-a-time to maximize
+fairness and minimize contention across multiple processes on a
+shared listen socket.  Accepting multiple connections at once may be
+useful in constrained deployments with few, heavily-loaded workers.
+Negative values enables a worker to accept all available clients at
+once, possibly starving others in the process.  C<-1> behaves like
+C<multi_accept yes> in nginx; while C<0> (the default) is
+C<multi_accept no> in nginx.  Positive values allow
+fine-tuning without the runaway behavior of C<-1>.
+
+This may be specified on a per-listener basis via the C<multi-accept=>
+per-listener directive (e.g. C<-l http://127.0.0.1?multi-accept=1>).
+
+Default: 0
+
 =back
 
 =head1 SIGNALS

diff --git a/lib/PublicInbox/Daemon.pm b/lib/PublicInbox/Daemon.pm
index 5743542131821020c7c577aee05d8fa36c16a9b0..30442227bdf87ebc3b59a61d85b047232c5e4071 100644 (file)
--- a/lib/PublicInbox/Daemon.pm
+++ b/lib/PublicInbox/Daemon.pm
@@ -136,6 +136,8 @@ sub load_mod ($;$$) {
        }
        my $err = $tlsd->{err};
        $tlsd->{warn_cb} = sub { print $err @_ }; # for local $SIG{__WARN__}
+       $opt->{'multi-accept'} and
+               $xn{'multi-accept'} = $opt->{'multi-accept'}->[-1];
        \%xn;
 }
 
@@ -167,6 +169,7 @@ EOF
                'u|user=s' => \$user,
                'g|group=s' => \$group,
                'D|daemonize' => \$daemonize,
+               'multi-accept=i' => \$PublicInbox::Listener::MULTI_ACCEPT,
                'cert=s' => \$default_cert,
                'key=s' => \$default_key,
                'help|h' => \(my $show_help),
@@ -251,7 +254,7 @@ EOF
                $s->blocking(0);
                my $sockname = sockname($s);
                warn "# bound $scheme://$sockname\n";
-               $xnetd->{$sockname} //= load_mod($scheme);
+               $xnetd->{$sockname} //= load_mod($scheme, $opt);
                $listener_names->{$sockname} = $s;
                push @listeners, $s;
        }
@@ -712,7 +715,8 @@ sub daemon_loop ($) {
                defer_accept($_, $tls_cb ? 'dataready' : $xn->{af_default});
 
                # this calls epoll_create:
-               PublicInbox::Listener->new($_, $tls_cb || $xn->{post_accept})
+               PublicInbox::Listener->new($_, $tls_cb || $xn->{post_accept},
+                                               $xn->{'multi-accept'})
        } @listeners;
        PublicInbox::DS::event_loop($sig, $oldset);
 }

diff --git a/lib/PublicInbox/Listener.pm b/lib/PublicInbox/Listener.pm
index 7cedc3493f872df866755e1c2c9d5d545df7c319..4669cf048187e2d428d0ed7ecdba52977829fd2b 100644 (file)
--- a/lib/PublicInbox/Listener.pm
+++ b/lib/PublicInbox/Listener.pm
@@ -1,14 +1,15 @@
-# Copyright (C) 2015-2021 all contributors <meta@public-inbox.org>
+# Copyright (C) all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 #
 # Used by -nntpd for listen sockets
 package PublicInbox::Listener;
-use strict;
+use v5.12;
 use parent 'PublicInbox::DS';
 use Socket qw(SOL_SOCKET SO_KEEPALIVE IPPROTO_TCP TCP_NODELAY);
 use IO::Handle;
 use PublicInbox::Syscall qw(EPOLLIN EPOLLEXCLUSIVE);
 use Errno qw(EAGAIN ECONNABORTED);
+our $MULTI_ACCEPT = 0;
 
 # Warn on transient errors, mostly resource limitations.
 # EINTR would indicate the failure to set NonBlocking in systemd or similar
@@ -16,37 +17,35 @@ my %ERR_WARN = map {;
        eval("Errno::$_()") => $_
 } qw(EMFILE ENFILE ENOBUFS ENOMEM EINTR);
 
-sub new ($$$) {
-       my ($class, $s, $cb) = @_;
+sub new {
+       my ($class, $s, $cb, $multi_accept) = @_;
        setsockopt($s, SOL_SOCKET, SO_KEEPALIVE, 1);
        setsockopt($s, IPPROTO_TCP, TCP_NODELAY, 1); # ignore errors on non-TCP
        listen($s, 2**31 - 1); # kernel will clamp
        my $self = bless { post_accept => $cb }, $class;
+       $self->{multi_accept} = $multi_accept //= $MULTI_ACCEPT;
        $self->SUPER::new($s, EPOLLIN|EPOLLEXCLUSIVE);
 }
 
 sub event_step {
        my ($self) = @_;
        my $sock = $self->{sock} or return;
-
-       # no loop here, we want to fairly distribute clients
-       # between multiple processes sharing the same socket
-       # XXX our event loop needs better granularity for
-       # a single accept() here to be, umm..., acceptable
-       # on high-traffic sites.
-       if (my $addr = accept(my $c, $sock)) {
-               IO::Handle::blocking($c, 0); # no accept4 :<
-               eval { $self->{post_accept}->($c, $addr, $sock) };
-               warn "E: $@\n" if $@;
-       } elsif ($! == EAGAIN || $! == ECONNABORTED) {
-               # EAGAIN is common and likely
-               # ECONNABORTED is common with bad connections
-               return;
-       } elsif (my $sym = $ERR_WARN{int($!)}) {
-               warn "W: accept(): $! ($sym)\n";
-       } else {
-               warn "BUG?: accept(): $!\n";
-       }
+       my $n = $self->{multi_accept};
+       do {
+               if (my $addr = accept(my $c, $sock)) {
+                       IO::Handle::blocking($c, 0); # no accept4 :<
+                       eval { $self->{post_accept}->($c, $addr, $sock) };
+                       warn "E: $@\n" if $@;
+               } elsif ($! == EAGAIN || $! == ECONNABORTED) {
+                       # EAGAIN is common and likely
+                       # ECONNABORTED is common with bad connections
+                       return;
+               } elsif (my $sym = $ERR_WARN{int($!)}) {
+                       warn "W: accept(): $! ($sym)\n";
+               } else {
+                       warn "BUG?: accept(): $!\n";
+               }
+       } while ($n--);
 }
 
 1;